1. Field of the Invention
The present invention is directed to new lacquer binders containing polyvinyl acetals and specific polyisocyanates with blocked isocyanate groups, a process for the formation of coatings on heat resistant substrates using coating compositions containing these binders, and the coated heat resistant substrates.
2. Description of the Prior Art
Stoving lacquers based on polyvinyl acetals, in particular for the insulation of electric conductors, have been known for a long time. U.S. Pat. No. 2,307,588, for example, describes lacquers based on polyvinyl acetals containing phenol resins as additives. In DE-AS 1,179,320, it is recommended to incorporate blocked polyisocyanates in such coating compounds for the purpose of improving the solvent resistance and long term heat resistance of the lacquers. According to DE-AS 1,153,127, a further improvement in such coating compounds consists of the additional use of melamine resins. This four fold combination of polyvinyl acetal (polyvinyl formal), blocked polyisocyanate, phenol resin and melamine resin, known as "Hermetic Formulation," has proven to be particularly useful in the manufacture of insulating wires for enclosed assemblies of refrigerating machines.
The last mentioned binder combination has proven its suitability for wire coating over several decades in use. It has now surprisingly been found, however, that the advantageous properties of these systems may be even further improved to a remarkable extent by using binders of the type according to the invention described below, which are primarily based on a polyvinyl acetal component and a specific polyisocyanate component.
By using the binders according to the invention described below, it is possible in particular to increase the abrasion resistance, the softening temperature and the increase in dielectric loss factor tan .delta. according to DIN 53 483. Conversely, the already advantageous properties of the systems known in the art may be obtained even when the lacquering speed is increased.
Another advantage of the binders according to this invention is that excellent lacquer properties are obtained even when polyvinyl acetals having a comparatively low molecular weight are used, i.e. polyvinyl acetals which in the form of a 10% by weight solution in furfural have a comparatively low viscosity. It was previously considered a fact of experience that satisfactory properties could only be obtained from starting materials having a sufficiently high molecular weight because the time available during the stoving process is not sufficient for forming a very large number of cross-linking points or cross-linking proceeds too spontaneously so that the solvents cannot evaporate without disturbing the film which is in the process of formation, with the result that a poor quality surface is obtained.
It has now been discovered that products with a lower molecular weight may be used enables lacquers with a higher solids content to be used, which is an advantage both economically and environmentally.